Sorta unrelated but here goes:
Last year I installed a Gear Vendors overdrive unit. To get it to fit, I had to cut out the exhaust crossover pipe. The engine sits in the car a bit out of level, so I tried to clock it after loosening the motor mount nuts. I laid a 2 foot level across the valve covers, but I had to spin the distributor a few degrees because the vacuum can was in the way....After the GV was in, I drove to the exhaust/muffler shop. The car ran terrible and was really slow turning over. It pinged & detonated even at light throttle. I pulled over about a mile after leaving and clocked the distributor until the car ran like before. That wasn't all though. I had also removed the valve cover breather. I noticed a CLANG then saw a chrome ball rolling behind me as I was rolling at about 50 mph. (What the heck was tha..... Oh, I know what it was)
The point is, it was good advice from the guy who suggested to try looking at the changes you made.